Posterior fossa tumors are tumors located at the posterior fossa region of the skull, i.e. the inner surface of the base of the skull. Cranial nerve involvement is common with posterior fossa tumors over a certain size. Acoustic neuromas are most common of the posterior fossa tumors and can involve hearing loss. Because of their location and neural involvement, surgery to correct such tumors is difficult. Despite advancements in diagnosis, microsurgical techniques, and neurotological techniques which enable more positive anatomical identification of facial nerves, loss of facial nerve function following acoustic neuroma resection is a significant risk. In this respect, nerves are very delicate, and even the best and most experienced surgeons, using the most sophisticated equipment known heretofore, encounter a considerable hazard that a nerve will be bruised, stretched or even severed during an operation.
Recent studies have shown that preservation of the facial nerve during acoustic neuroma resection may be enhanced by the use of intraoperative electrical stimulation to assist in locating nerves. Very broadly stated, this procedure involves inserting recording electrodes directly within cranial muscles controlled by the nerve of interest. An electrical probe is then applied near the area where the subject nerve is believed to be located. If the probe contacts, or is reasonably near the nerve, the signal applied thereto is transmitted through the nerve to excite the related muscles. Excitement of the muscles causes an electrical impulse to be generated therein, which impulse is transferred to the recording electrodes which provide, by suitable means, an indication to the surgeon as to the location of the nerve.
While intraoperative electrical stimulation has been of benefit in the localization and preservation of facial nerves during various surgical procedures, the accuracy and reliability of such stimulation depends upon eliminating sources of false stimulation. A major source of false stimulation is the shunting of the electrical stimulus away from the intended area through body fluids. In this respect, during acoustic neuroma surgery the surgical area is variably bathed in cerebrospinal fluid (CSF), which is a clear, colorless body fluid containing electrolytes and is capable of conducting electrical current. Heretofore, stimulus probes were comprised of standard bare wire or tapered metal rods which were touched to the area to be stimulated. These probes allow electrical contact with the electrolyte fluid such that the electrical stimulus may spread along parallel channels. Such spread of the stimulus reduces electrical current flow through the point of contact with the tissue intended for stimulation, which may result in false stimulation and thus affect the accuracy of the procedure. It has been suggested that one solution to this problem is to increase the intensity level of the electrical stimulus such that the neural response to stimulation occurs despite such shunting. Increased stimulus levels however increase the possibility of tissue damage. Further, the stimulus may spread through inactive tissue and reach active tissue at a level sufficient to produce a false response which would likewise affect the accuracy of the procedure.
These and other problems are overcome by a preferred embodiment of the present invention which provides an electrical stimulus probe which reduces shunting of the electrical stimulus through body fluids and provides a higher degree of spatial selectivity.